1. Field of the Invention
The present invention is directed to a nuclear magnetic resonance tomography apparatus of the type wherein at least one gradient coil is connected with at least one capacitor to form a resonant circuit, which is connected to a gradient amplifier that is driven according to a predetermined time function.
2. Description of the Prior Art
Nuclear magnetic resonance tomography systems are known which are operated with a pulse sequence, with a read-out sequence occurring after each excitation per scan. At least two gradients disposed perpendicularly relative to each other are generated for location coding, with the resulting signals being digitized and written in the k-space in a raw data matrix. An image is then acquired from the raw data matrix by two-dimensional Fourier transformation. It is known to connect at least one of the gradient coils with at least one capacitor to form a resonant circuit. The resonant circuit is connected to a gradient amplifier which is driven according to a predetermined time function. Each gradient pulse during the read-out sequence consists of a leading edge and a trailing edge, and a constant part therebetween. The leading and trailing edges are generated during the resonant operation of the gradient coil and the constant part is non-resonantly generated by the gradient amplifier.
Such an arrangement is disclosed by European Application 0 227 411. The resonant operation of the gradient coils allows short rise and decay times of the gradient pulses to be achieved. Such short rise and decay times could not otherwise be achieved, or could only be achieved with excessive outlay in the gradient amplifier.
Short switching times of the gradients are necessary, for example, for imaging according to the EPI method, which shall be described briefly below with reference to FIGS. 1 through 7 for explaining the problem to which the subject matter herein is directed. A more detailed description of the EPI method may be found in European Application 0 076 054.
FIGS. 1 through 6 show an example of a pulse sequence employed in the EPI method. The examination subject is simultaneously charged with a radio-frequency excitation pulse RF together with a gradient SS in the z-direction. Nuclear spins in a selected slice of the examination subject are thereby excited. Subsequently, the direction of the gradient SS is inverted, so that the negative portion of gradient SS cancels the dephasing of the nuclear spins caused by the positive portion of the gradient SS.
After the excitation, a phase-coding gradient PC and a read-out gradient RO are generated. There are various possibilities for the shape of these gradients. As a first example, FIG. 3 shows a phase-coding gradient PC which is continuously activated during the read-out phase. As an alternative, FIG. 4 shows a phase-coding gradient PC' consisting of short individual pulses ("blips") which are activated at each polarity change of the read-out gradient RO. Each phase-coding gradient PC and PC' is preceded by a pre-phasing gradient PCV in the negative phase-coding direction. The read-out gradient RO, shown in FIG. 5, has constantly changing polarity, as a result of which the nuclear spins are alternatingly dephased and rephased, so that a sequence of signals S arises, as shown in FIG. 6. In a single excitation, so many signals are acquired that the entire Fourier k-space is scanned, i.e., the existing data are adequate for reconstructing a complete tomogram. An extremely fast switching of the read-out gradient RO with a high amplitude is required for this purpose, which is no able to be achieved with the square-wave pulses which are usually employed in MR imaging. A standard solution to this problem is to operate the gradient coil which generates the read-out gradient RO in a resonant circuit, so that the read-out gradient RO has a sinusoidal shape.
The arising nuclear magnetic resonance signals S are scanned in the time domain, are digitized, and the numerical values acquired in this manner are entered into a raw data matrix. The raw data matrix can be considered as a measured data space, such as a measured data plane in the two-dimensional case shown in the exemplary embodiment. This measured data space is referred to as k-space in nuclear magnetic resonance tomography. The position of the measured data in the k-space is schematically shown in FIG. 7 for a phase-coding gradient PC as shown in FIG. 3, and is schematically shown in FIG. 8 for a phase-coding gradient PC' as shown in FIG. 4. Information about the spacial origin of the signal contributions, which is necessary for imaging, is coded in the phase factors, with the relationship between the locus space (i.e. the image) and the k-space existing mathematically via a two-dimensional Fourier transformation. This is represented by the equation: EQU S(k.sub.x, k.sub.y)=.intg..intg..zeta.(x,y)e.sup.i(k.sbsp.x.sup.x+k.sbsp.y.sup.y) dx dy,
for which the following definitions apply: ##EQU1## and wherein .zeta. is the nuclear spin density, .gamma. is the gyromagnetic ratio, G.sub.x is the value of the read-out gradient RO and G.sub.y is the value of the phase-coding gradient PC (or PC').
Extremely high amplitudes for locus coding of the MR signals are necessary in the EPI method. These high gradient amplitudes must be switched on and off in short time intervals so that the necessary information can be acquired before the nuclear magnetic resonance signal decays. If it is assumed that one millisecond is required for a projection (i.e., for a single signal under a discrete pulse of the read-out gradient RO), then an overall read-out time T.sub.acq of 128 ms is needed for a 128.times.128 image matrix. If conventional square-wave pulses having a duration of .delta.t=1 ms were used and a field of view FOV of 40 is assumed, then typical gradient amplitudes G.sub.R of ##EQU2## derive for the read-out pulse RO for square-wave pulses. For trapezoidal pulses having a rise time of T.sub.rise =0.5 ms and read-out of the signals on the ramps of those pulses, even larger gradient pulses G.sub.T devive as follows: ##EQU3## As can be seen from the above equations, the demands made on the necessary gradient amplitude are more easier to achieve with shorter rise times. By contrast, the demands made on the switching speed of the gradient amplifier become greater with decreasing rise time. If it is assumed that a current I.sub.max is required for achieving the maximum gradient strength G.sub.max, the voltage U required due to the inductivity L of the gradient coil is calculated as: ##EQU4## The ohmic voltage drop across the gradient coil is not yet taken into consideration. For an inductivity of the gradient coil of 1 mH and a maximum current I.sub.max of 200 A, the voltage required at the output of the gradient amplifier would assume the following values dependent on the rise time T.sub.rise of the gradient current:
______________________________________ T.sub.rise = 0.5 ms U = 400 V T.sub.rise = 0.25 ms U = 500 V T.sub.rise = 0.1 ms U = 2,000 V. ______________________________________
These are demands can only be achieved, without a resonant circuit, by means of a substantial circuit outlay typically by parallel connections and series connections of modular gradient amplifiers.
The problem of achieving shot switching times can be more simply solved by operating the gradient coil in question together with a capacity in a resonant circuit, thereby producing the sinusoidal curve of the read-out gradient RO as shown in FIG. 5. A disadvantage of this approach, however, is that equi-distant sampling in the k-space is not obtained when sampling the signal at chronologically constant intervals. In order to avoid this problem, it is proposed in the aforementioned European Application 0 227 411 only to generate the rising and decaying edges of the gradient pulses by means of the resonant circuit, i.e., only these edges are in sinusoid form, and to provide the gradient pulse with a constant value between these rising and decaying edges. In this approach, however, each of the rise time and decay time occupies one-fourth of the duration of the oscillation in which is contained, which is relatively long.
The principles of generating a gradient pulse in a known manner using a resonant circuit are described in greater detail below with reference to FIGS. 9 through 14.
A series resonant circuit is schematically shown FIG. 9 having a capacitor C and a gradient coil G which is connected via a switch S1 to a gradient amplifier PSU. The series circuit of the switch S1 and the capacitor C is bridged with a further switch S0. Further, the capacitor C is connectable to a charging voltage source U.sub.lad via a switch S2. A circuit of this type is disclosed by European Application 0 429 715.
The drive of the series resonant circuit is set forth below for a single, unipolar gradient pulse with reference to the curves shown in FIGS. 10 through 14. FIG. 10 shows the curve of the current I.sub.G through the gradient coil G. FIG. 11 shows the curve of the voltage U.sub.C across the capacitor C. FIG. 12 is a curve of the voltage U.sub.G across the gradient coil G. FIGS. 13 and 14 respectively show the switching states of the switches S0 and S1.
First the capacitor C is charged with the charging voltage source U.sub.lad until it has reached the maximum voltage U.sub.c.sup.max at a time t.sub.C. The energy E=1/2U.multidot.C.sup.2 is thus stored in the capacitor C. The switch S2 is closed during the charging of the capacitor C. The switch S1 is subsequently opened at a time t0. The series resonant circuit composed of the capacitor C and the gradient coil G thus begins to oscillate, i.e., the current I.sub.G rises with a sinusoidal edge. The voltage U.sub.C across the capacitor C drops to zero at time t1. The switch SO is now closed. The gradient amplifier PSU thus supplies current via the switch SO directly to the gradient coil G, and the capacitor C remains discharged. The resonant circuit energy is now stored in the form of current in the gradient coil G. While the capacitor C is discharged, the state of the switch S1 is of no consequence, and is therefore shown shaded in FIG. 14.
The switch SO is again opened at time t2, and the energy of the gradient coil G is now transferred again into the capacitor C, which charges to a negative voltage to the maximum value U.sub.C.sup.max. At time t3, all of the energy is again stored in the capacitor C, and the current through the gradient coil G is thus zero. The trailing edge of the gradient current I.sub.G again has the shape of a quarter sine wave.
In the arrangement described above, the rise and decay times (t1-t0 and t3-t2) of the gradient current I.sub.G are defined by one-fourth of the period of a sinusoidal oscillation of the gradient current I.sub.G, which is in turn prescribed by the resonant frequency of the series resonant circuit.